1. Field of the Invention
The invention is related to a chemically toughened ultrathin glass, in particular, related to a high strength flexible glass, and more particularly, related to a flexible glass used for flexible electronics for flexible printing, sensors for touch panels, substrates for thin film cells, mobile electronic devices, semiconductor interposers, bendable displays, solar cells or other applications in need of high chemical stability, temperature stability, low gas permeability as well as flexibility and low thickness. Besides consumer and industrial electronics the current invention could also be used for protection applications in industrial manufacture or metrology.
2. Description of Related Art
Thin glass with different compositions is suitable substrate materials for many applications where chemical and physical properties such as transparency, chemical and thermal resistance are of great importance. For example, alkaline free glass such as AF32®, AF37®, AF45® available from SCHOTT can be used for display panels, and wafers as electronic packaging materials. Borosilicate glass also can be used for fire protection, thin and thick film sensors, and lab wares such as micro-mechanical components and lithographic masks.
Ultrathin glass is typically applied in electronics applications, such as films and sensors. At the present time, the increasing demands for new functionalities of products and exploiting new and broad applications call for thinner and lighter glass substrates with new properties such as flexibility.
Typically, thin glass is made by grinding a thicker glass such as borosilicate glass, however, glass sheets with a thicknesses lower than 0.5 mm would be difficult to be made via grinding and polishing larger glass sheets, or can be produced only under extremely strict conditions. Glass thinner than 0.3 mm, or even with a thickness of 0.1 mm, such as D263®, MEMpax®, available from SCHOTT, can be produced by down-drawing. Also, soda lime glass with a thickness of 0.1 mm can be produced by special float process.
The main challenge for applying ultrathin glass substrates to electronics lies in the treatability of thin glass sheets. Normally, there is lack of ductility for glass, and the possibility of breakage largely depends on the mechanical strength of the sheet itself. For thin glass, some methods have been proposed. U.S. Pat. No. 6,815,070 (Mauch et al.) proposed coating thin glass with organic or polymer films to improve the breaking strength of glass. Nevertheless, there yet exist some disadvantages for this method, for example, the improvement in strength is not sufficient and then some other sophisticated processes must be adopted when glass sheets cut. In addition, the polymer coating would exert a negative influence on the thermal durability and optical property of glass sheets.
In addition, chemical toughening is a well-known process to increase strength of a thicker glass like soda lime glass or aluminosilicate (AS) glass that is used as cover glass for display applications, for example. In this circumstance, the surface compressive stresses (CS) are typically between 600 and 1,000 MPa and the depth of the ion-exchange layer is typically bigger than 30 μm, preferably bigger than 40 μm. For safety protection applications in transportation or aviation, AS Glass has a exchange layer bigger than 100 μm. Normally, a glass having both high CS and high DoL is expected for all the applications when the glass thickness ranges from about 0.5 mm to 10 mm. However, for ultrathin glass, the high CS at a high DoL will result in self breakage due to high central tensile stress of glass, therefore, new parameters should be controlled for ultrathin glass which are different from those used for cover glass.
Studies have been conducted on chemical toughening of glass in a great number of inventions, for example, US 2010/0009154 describes a glass of 0.5 mm or thicker with an outer region of compressive stress, the outer region has a depth of at least 50 μm and the compressive stress is at least higher than 200 MPa, the step of forming the central tensile stress (CT) and the compressive stress in the surface region comprises successively immersing at least a portion of the glass in a plurality of ion exchange baths, and the glass thus obtained could be used for consumer electronic. The parameters and requirements for production of such glass do not apply to production of ultrathin glass because the central tension would be so high as to cause self-breakage of glass.
US 2011/0281093 describes a strengthened glass with damage resistant ability, the strengthened glass article has a first and second compressive stress surface portions opposite each other bound to a tensile stress core portion, with the first surface portion having a higher level of compressive surface stress than the second surface portion for the purpose of improving the resistance to surface damage. The compressive stress surface portions are provided by lamination, ion-exchange, thermal tempering, or combinations thereof to control the stress profile and to limit the breakage energies of articles.
WO 11/149694 discloses an antireflection coating glass which is chemically strengthened, with a selected coating on at least one of the surfaces of the glass article, where the coating is selected from the group consisting of an antireflection and/or an antiglare coating, and said coating contains at least 5 wt % of potassium oxide.
US 2009/197048 sets forth that a chemically toughened glass is bonded with a functional coating in order to serve as a cover plate. The glass article has a surface compressive stress of at least about 200 MPa, a surface compressive layer depth in the range of 20 μm to 80 μm, and has an amphiphobic fluorine-based surface layer chemically bonded to the surface of the glass article to form a coated glass article.
In U.S. Pat. No. 8,232,218, heat treatment has been used for improving the effects of chemical toughening of glass. The glass article has an anneal point and a strain point, wherein the glass article is quenched from a first temperature that is higher than the anneal point of the glass article to a second temperature that is lower than the strain point. The fast cooled glass will have a higher compressive stress and a thicker ion-exchange layer after chemical toughening.
In US 2012/0048604, the ion-exchanged ultrathin aluminosilicate or aluminoborosilicate glass sheet is used as interposer panels for electronics. The interposer panels include a glass substrate core formed from ion-exchange of glass. The coefficient of thermal expansion (CTE) is set to match that of semiconductors and metallic materials and the like. However, in this patent application, a compressive stress higher than 200 MPa on the surface layer is required, and the depth of the layer is apt to become too deep for aluminosilicate or aluminoborosilicate glass. The above factors make ultrathin glass difficult for practical use. Besides, the flexibility of glass and how to improve it are not considered. In fact, the flexibility is the most important factor for its application for the ultrathin glass with a thickness equal or less than 0.3 mm. In addition, chemical strengthening process requires immersion of a glass substrate into a salt bath at high temperature, and the process would require the glass itself to possess high thermal shocking resistance. Throughout the disclosure of the invention, it is not discussed how to adjust the glass composition and the relevant functions to meet the said requirements.
For example, self-breakage is a serious problem for aluminosilicate glass, because the high CTE of aluminosilicate glass lowers the thermal shock resistance, increasing the possibility of breakage for thin glass during toughening and other treatments. On the other hand, most aluminosilicate glasses have a higher CTE that does not match that of semiconductor electronic devices, increasing the difficulty for treatment and application.
The current invention has successfully solved the above technical problems present in the prior art via providing a novel flexible glass substrate whose flexibility can be enhanced by chemical toughening. Meanwhile, the composition of ultrathin flexible glass has been specially designed to acquire excellent thermal shocking resistance for chemical toughening and practical use. Another important fact is that the flexible ultrathin glass of the present invention is characterized by having lower compressive stress and shallower depth of compressive stress layer compared to other glasses after being chemically toughened. Such properties make the glass sheet of the present invention more suitable for practical processing.